Humm in my preamp...

Bryan · 19th November 2005, 02:37 AM

Hey All,

So, I have rebuilt my preamp (seen in 12B4A Linestage thread) in new enclosures and I now have some hefty hummmmmmm.

I've looked over my grounding scheme, and it looks fine. I think the problem may exist from the PS chassis, and the chokes being places too near each other.

So, time to ask the experts...

First things first. When I look at the ripple, I measure 5.0 and 4.1 mV on the L and R channel output. I can trace this back to the plate. What is more interesting is the ripple at the last filter cap before the ccs which loads the plate. It dances around at values between 8 and 14 mV, and is not steady. After the CCS, it is staedy at the 5.0 and 4.1 mV values.

Any thoughts?

Geek · 19th November 2005, 02:57 AM

Sounds to me like it may be 1) poor chassis layout or 2) induction throught he cassis.

Did you use steel or aluminum?

Bryan · 19th November 2005, 03:43 AM

Aluminum.

To clarify (or complicate matters) there are two chassis. PS and Audio. Both are built on aluminum top plates which sit on wood chassis. Both top plates are grounded.

Interesting enough, I just pulled the regulator tubes (thus eliminating B+ from the valves) and I still have the hummmm. Would this lead one to believe the humm is coming from the heater circuit? I'll poke around there and see if I can uncover anything....

Shoog · 19th November 2005, 08:30 AM

AC heaters are very unlikely to induce that much hum. Almost all valves cope very well with AC heaters.
Its much more likely to be induced hum from poor layout. My last Valve headphone amp had real issues with hum, which all turned out to be induced and ground loops. I just completed a valve phone amp and absolutely no hum first time.

Its really a matter of search the net for guidance on eliminating earth loop hums and trying everything systamatically one at a time.

Shoog

dhaen · 19th November 2005, 11:31 AM

Quote:

Originally posted by Bryan
snip..
Interesting enough, I just pulled the regulator tubes (thus eliminating B+ from the valves) and I still have the hummmm. ..snip...

Aren't your regulators shunt? If they are, pulling them doesn't remove B+...

Bryan · 19th November 2005, 01:08 PM

This is in fact true. I realized this error last evening after I typed the reply, but could not find the "edit" tab on the webpage...

I'm going to tweak with the wiring layouts and tripple check my grounding. It must be something simple that I am overlooking.

I'll report back my findings after I finish poking around.

Thanks,

Bryan

19th November 2005, 02:37 AM	#1
Bryan diyAudio Member Join Date: Jun 2003 Location: Reading MA	Humm in my preamp... Hey All, So, I have rebuilt my preamp (seen in 12B4A Linestage thread) in new enclosures and I now have some hefty hummmmmmm. I've looked over my grounding scheme, and it looks fine. I think the problem may exist from the PS chassis, and the chokes being places too near each other. So, time to ask the experts... First things first. When I look at the ripple, I measure 5.0 and 4.1 mV on the L and R channel output. I can trace this back to the plate. What is more interesting is the ripple at the last filter cap before the ccs which loads the plate. It dances around at values between 8 and 14 mV, and is not steady. After the CCS, it is staedy at the 5.0 and 4.1 mV values. Any thoughts? __________________ Dont take life too seriously, you will never get out alive!

19th November 2005, 03:43 AM	#3
Bryan diyAudio Member Join Date: Jun 2003 Location: Reading MA	Aluminum. To clarify (or complicate matters) there are two chassis. PS and Audio. Both are built on aluminum top plates which sit on wood chassis. Both top plates are grounded. Interesting enough, I just pulled the regulator tubes (thus eliminating B+ from the valves) and I still have the hummmm. Would this lead one to believe the humm is coming from the heater circuit? I'll poke around there and see if I can uncover anything.... __________________ Dont take life too seriously, you will never get out alive!

19th November 2005, 01:08 PM	#6
Bryan diyAudio Member Join Date: Jun 2003 Location: Reading MA	This is in fact true. I realized this error last evening after I typed the reply, but could not find the "edit" tab on the webpage... I'm going to tweak with the wiring layouts and tripple check my grounding. It must be something simple that I am overlooking. I'll report back my findings after I finish poking around. Thanks, Bryan __________________ Dont take life too seriously, you will never get out alive!

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19th November 2005, 02:57 AM	#2
Geek diyAudio Member Join Date: Sep 2004	Sounds to me like it may be 1) poor chassis layout or 2) induction throught he cassis. Did you use steel or aluminum?

19th November 2005, 08:30 AM	#4
Shoog diyAudio Member Join Date: Aug 2002 Location: Eire	AC heaters are very unlikely to induce that much hum. Almost all valves cope very well with AC heaters. Its much more likely to be induced hum from poor layout. My last Valve headphone amp had real issues with hum, which all turned out to be induced and ground loops. I just completed a valve phone amp and absolutely no hum first time. Its really a matter of search the net for guidance on eliminating earth loop hums and trying everything systamatically one at a time. Shoog

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